The present invention relates to a sample CD (critical dimension) measurement system, and particularly to a sample CD measurement system adapted for CD measurement of a device pattern in production of a semiconductor device, a magnetic storage device, an image pickup device, a display device, or the like.
Main examples of sample CD measurement systems which are the subject of the present invention include a scanning electron microscope (SEM, especially, a CD measurement SEM), a laser microscope, an i-ray microscope, a scanning interatomic force microscope, etc. The production of semiconductors will be described as an example of an typical application field of a sample CD measurement system. In the production of semiconductors, a CD measurement SEM (CD-SEM) is widely used for the CD measurement of a pattern. An example of the CD-SEM will be described below.
FIG. 1 shows the basic theory and configuration of a CD-SEM. An electron beam 2 emitted from an electron beam source 1 is narrowed by a condenser lens 3 and an objective lens 4 so as to be focused on a surface of a wafer 5 which is a sample. At the same time, the path of the electron beam 2 is deflected by a deflector 6 so that the electron beam 2 scans the surface of the wafer 5 two-dimensionally. On the other hand, secondary electrons 7 are emitted from a wafer portion exposed to the electron beam 2. These secondary electrons are detected by a secondary electron detector 8 and converted into an electric signal. The electric signal is converted into a digital signal by an A/D converter and then, the digital signal is stored in an image memory 10. The signal stored in the image memory 10 is processed by an image processor 11 so as to be used for intensity modulation of a display 12. The display 12 is scanned in the same way as the scanning of the electron beam 2 on the surface of the wafer, so that a sample image (SEM image) is formed on the display.
A pattern form is inspected with the CD-SEM, for example, by the following procedure.
A sheet 5 of test wafer taken out from a wafer cassette 13 is pre-aligned by reference to an orientation flat, a notch, or the like, of the wafer. After the pre-alignment, the wafer 5 is carried and put onto an XY stage 15 in a sample chamber 14 kept in a vacuum. The wafer 5 loaded on the XY stage 15 is wafer-aligned by using an optical microscope 16 attached to an upper portion of the sample chamber 14. Specifically, an image obtained by the optical microscope 16 is converted into an electric signal by an image pickup device such as a CCD, or the like. The electric signal is converted into a digital signal by an A/D converter 17. The digital signal is stored in the image memory 10. The signal stored in the image memory 10 is coupled to a display 12 through an image processor 11, so that the optical microscope image is displayed on the display 12. An alignment pattern formed on the wafer 5 is used for the wafer alignment. A field of view in the optical microscope image of the alignment pattern magnified by a magnification factor of the order of hundreds of times is compared with a field of view in a reference image of an alignment pattern registered in advance, so that the coordinates of the stage position are corrected to make the fields of view in the two images coincide with each other. After the alignment, the wafer is moved on the stage to a desired measurement site. After the wafer is moved to the measurement site, a scanning electron beam is radiated onto the wafer to form an SEM image of the measurement portion. This SEM image is processed so that the CD of a pattern in a designated portion is obtained on the basis of the image intensity profile of the measurement portion.
Incidentally, the storage/reading of the image signal, the processing of the image signal, and so on, are controlled by a computer and controller 18.
As the pattern becomes finer and denser, the needs of measuring not only the CD of a line pattern or a circular hole pattern but also the CD of an irregular curved pattern increase. Not only this makes it difficult to perform positioning of the designated measurement point accurately but this also raises a requirement to measure an oblique shape in an oblique direction, for example, as represented by the designated measurement site 1 in FIG. 4.